The toxicity of diphtheria toxin is believed to result from its activity in inhibiting protein synthesis in sensitive cells. The enzymic activity which is responsible for this inhibition has been identified, and some very interesting structure-activity relationships in the toxin molecule have been determined. Specifically it has been found that the expression of the crucial enzymic activity depends upon both limited proteolysis and reduction of the toxin, which results in the release of an enzymically active fragment (fragment A). We propose here to study in greater detail the structure and enzymology of diphtheria toxin and to explore other less well known areas of its action. In extending our studies on the structure and activity of the toxin, we propose to do further work on the kinetics of the enzymic reaction, the primary structure of the toxin, the interaction of the toxin and the enzymically active fragment with substrates, and other related problems. In addition we propose to explore ways to study the attachment of the toxin to cell surfaces and the role of EF 2 in protein synthesis. We will explore in particular the possibility of using bifunctional, cross-linking reagents to study these problems.